1. Field of the Invention
The present invention relates to a synchronization and rectification type voltage going up and down DC-DC converter, and in particular, to that capable of changing and setting an output voltage close to zero as the lowest.
2. Discussion of the Background Art
A compact mobile instrument, such as a mobile phone, etc., is widely spreading, recently. As a power for the compact mobile instrument, a secondary battery is generally used. To downsize and elongate a usage time period of the secondary battery as long as possible, it has been attempted to sophisticate the battery while saving power.
To use longer while downsizing the battery, it is preferable to broaden an available voltage arrange of the battery as much as possible.
Then, a voltage up-and-down DC-DC converter having a wide range of input voltages is utilized in a power source circuit.
Due to the wide range, the voltage up-and-down DC-DC converter has an advantage of handling various input power sources, such as a battery, an AC adapter, etc.
The voltage up-and-down DC-DC converter with an inductor is widely used as a power source in an instrument having a battery because of its circuit simplicity and high efficiency as described in Japanese Patent Application Laid Open Nos. 2004-328964 and 2005-198411.
FIG. 2 illustrates only an output circuit section of the voltage up-and-down DC-DC converter of the Japanese Patent Application 2004-328964.
As shown, M1 to M4 denote switching transistors (PMOS) for voltage going down use, a synchronization and rectification use transistor (NMOS) for voltage going down use, a switching transistor (NMOS) for voltage going up use, and a synchronization and rectification use transistor (NMOS) for voltage going up use, respectively.
Each of gates of the transistors M1 to M4 is connected to a predriver 11, and receives a control signal from a control circuit, not shown.
When an input voltage goes down, the switching transistor M3 is always turned off, because its gate receives a low level signal. Whereas the synchronization and rectification use transistor M4 is always turned on at the time, because its gate receives a high level signal.
Each of the transistors M1 and M2 receives, through its gates, low and high level signals in a pulse state to be turned on and off in accordance with an output voltage Vout. However, the transistors M1 and M2 aren't turned on, simultaneously.
When an input voltage goes up, the transistor M1 is always turned on, because its gate receives a low level signal. Whereas the transistor M2 is always turned off at the time, because its gate receives a low level signal. Each of the transistors M3 and M4 receives, through its gates, low and high level signals in a pulse state to be turned on and off in accordance with an output voltage. The transistors M3 and M4 are not simultaneously turned on, however.
Since the NMOS transistor is utilized for the synchronization and the rectification use transistor M4 in this output circuit, a gate voltage higher than the sum of an output voltage “Vout” and a threshold voltage of the NMOS transistor M4 is needed to turn on the transistor. Accordingly, a power source for the predriver 11 needs a higher voltage than the gate voltage.
Thus, no problem occurs when a power is supplied to the predriver 11 from the input voltage “Vin” higher than an output voltage “Vout”, specifically when an input voltage goes down. However, when the input voltage “Vin” is lower than the output voltage “Vout”, specifically when an input voltage goes up, the synchronization and rectification use transistor M4 can't be turned on. Thus, a power should be supplied from another source higher than the output voltage Vout.
Thus, a higher voltage than the output voltage Vout is created as a power source for the predriver by pressurizing an input voltage Vin by means of a charge pump circuit or the like.
FIG. 3 illustrates only an output circuit section of the voltage up-and-down DC-DC converter described in the Japanese Patent Application 2004-328964, wherein like reference numerals and marks designate corresponding parts in FIG. 2. A significant difference from FIG. 2 is that a PMOS transistor for a synchronization and rectification use transistor M4 is employed for voltage going up use.
Each of gates of a voltage going down use switching transistor M1 (PMOS) and a voltage going down circuit use synchronization and rectification use transistor M2 (NOMOS) is connected to a voltage going down use driver 21. Similarly, each of gates of the voltage going up use switching transistor M3 (NMOS) and the voltage going up use synchronization and rectification use transistor M4 (POMOS) is connected to a voltage going up circuit use driver 22. Similar to those as described with reference to FIG. 2, a control circuit, not shown, applies a control signal to each of the gates in accordance with a situation if an input voltage goes up or down.
Since the voltage going up use synchronization and rectification use transistor M4 is formed from the PMOS transistor, a high voltage is not needed for a gate voltage to turn on the transistor M4.
However, when an input voltage goes down and an output voltage Vout is lower than a threshold voltage of the synchronization and rectification use transistor M4 (PMOS), the transistor M4 is not turned on even if the gate voltage of the synchronization and rectification use transistor M4 is decreased down to zero. Thus, such a DC-DC converter is not suitable where an input voltage goes down and the output voltage Vout is low.